1. Field of the Invention
The present invention relates to the manufacture of semiconductor wafers, and in particular to a system for safeguarding fab operators and workpieces such as semiconductor wafers from harm as the workpieces are transported between tools by a workpiece transport assembly.
2. Description of Related Art
A SMIF system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transport, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers, and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
A SMIF system has three main components: (1) minimum volume, sealed pods used for storing and transporting wafers and/or wafer cassettes; (2) an input/output (I/O) minienvironment located on a semiconductor processing tool to provide a miniature clean space (upon being filled with clean air) in which exposed wafers and/or wafer cassettes may be transferred to and from the interior of the processing tool; and (3) an interface for transferring the wafers and/or wafer cassettes between the SMIF pods and the SMIF minienvironment without exposure of the wafers or cassettes to particulates. Further details of one proposed SMIF system are described in the paper entitled xe2x80x9cSMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING,xe2x80x9d by Mihir Parikh and Ulrich Kaempf, Solid State Technology, July 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.02 microns (xcexcm) to above 200 xcexcm. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half xcexcm and under. Unwanted contamination particles which have geometries measuring greater than 0.1 xcexcm substantially interfere with 1 xcexcm geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.1 xcexcm and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles and molecular contaminants become of interest.
A SMIF system includes a minimum volume, sealed pod used for storing and transporting wafers. Within a wafer fab, a first automated transport system is provided for transferring the SMIF pods from one processing tool bay to another (interbay delivery systems), and a second automated transport system is provided for transferring the pods around within each particular bay (intrabay delivery systems). Each tool bay, typically on the order of about eighty feet long, consists in general of a number of processing tools for performing various wafer fabrication functions, and at least one stocker, where the pods may be stored before or after processing. Additionally, as a pod is generally transferred to several processing tools within a particular bay, the pod may be stored in the stocker between processes. A stocker is typically a large unit having a plurality of shelves on which the pods may be stored, and a transport system for transferring pods into and out of the stocker, and for moving pods around within the stocker.
U.S. Pat. No. 5,980,183 to Fosnight, previously incorporated by reference, discloses an intrabay pod storage and transport system comprising a plurality of pod support surfaces, or nests, distributed throughout the tool bay and mounted to the sides, on the front and/or above the tools within the bay. The intrabay pod storage and transport system further includes a transport assembly comprised of at least one robotic pod gripper mounted on horizontal and vertical rails to enable pod transport in the X-Z plane, i.e., along the length and height of the tool bay, to transport the pods between the various storage nests and load ports for the process tool.
Typically, some processing tools within a tool bay are high throughput tools which are capable of performing their particular wafer process at a relatively higher rate than other processing tools. Additionally, some tools within a bay are metrology tools, which in general monitor or test a single wafer from within a pod of wafers. A pod may store, for example, 25 wafers. If a normal throughput tool can process 50 wafers in an hour, the transport system need only supply 2 pods per hour to that tool. However, for metrology tools which can process 50 wafers in an hour, but only use one wafer per pod, 50 pods must be provided to the metrology tool in an hour to keep the tool from sitting idle.
In order to accommodate high throughput and metrology tools, it is known to include a local tool buffer adjacent the tool port of high throughput and metrology tools, so that pods may be stored locally adjacent such tools and quickly transferred to these tools without having to constantly retrieve a pod from the remotely located stocker. Such local tool buffers are generally configured adjacent the high throughput and metrology tools, and include shelves for storing pods, and a transport system for transferring pods to, from, and within the local tool buffer.
U.S. patent application Ser. No. 08/891,543 to Bonora et al., previously incorporated by reference, discloses a local tool buffer including a plurality of nests proximate to high throughput tools and a local transport assembly comprised of at least one robotic gripper capable of transporting the pods in the X-Z plane to transport the pods between the various storage nests and the high throughput process tool at which the nests are located.
Pod storage and transport systems such as those disclosed in U.S. Pat. No. 5,980,183 and U.S. patent application Ser. No. 08/891,543 provide several advantages with respect for example to flexibility of tool bay design and refit, the ease with which the storage and transport system may be scaled to fit tool bays of different sizes, and improved pod delivery times and throughput. However, the coexistence of pod storage and transport systems in a tool bay with the human fab operators within the tool bay present several concerns which must be addressed.
A first concern is to avoid injury to fab operators as a result of collision with the pod transport assembly or pods carried thereby as the transport assembly transports the pods within the tool bay. Fab operators commonly work within the tool bays to monitor the operation of the tools, manually transport pods to and from tools and perform maintenance on the tools within the bay. Semiconductor Equipment and Handling International (SEMI) Draft Document 2843D requires that all high speed horizontal transport of pods by the transport assembly occur above 2135 mm from the floor. Alternatively, the system could be configured for the OSHA minimum egress height of 7xe2x80x26xe2x80x3. However, the pod transport assembly must be able to lower into the areas occupied by the fab operators (which areas are referred to herein as human/automation common areas) to transfer pods to and from load ports, typically located at 900 mm from the floor, and storage nests in the common areas. Therefore, there is a danger of collision between the transport assembly or pods carried thereby and an operator in these human/automation common areas. There are known safety procedures in place to prevent an operator from being hurt by the moving transport assembly, but these procedures can be ignored by an operator, thus potentially placing the operator in harm""s way.
Another concern with the coexistence of the pod storage and transport system and the fab operators is the potential for damage to the workpieces carried in the pods. When pods are being transported through the human/automation common areas, the moving transport assembly or pods may collide with an operator as described above, or an operator may otherwise try to manually access a pod carried by the robotic gripper. Such contact may result in an abrupt jolt to the workpiece which can cause them to break. The contact may also result in the workpieces being dislodged from their storage shelves within the pod, which may subsequently result in damage to one or more workpieces at a load port when a workpiece handling robot encounters a workpiece at an unexpected position. Contact between the operator and transport assembly or pod can alternatively result in the pod disengaging from the robotic gripper entirely. Currently, each pod may carry in excess of $1 million worth of wafers or other workpieces, and any such damage to the workpieces as a result of contact with an operator can result in significant losses.
A further concern regarding the coexistence of the pod storage and transport system and fab operators is the ease of tool maintenance and the effect of tool maintenance on the overall operation of the transport assembly. Ideally, tool maintenance and the addition or replacement of tools within the tool bay would have no effect on pod delivery to other areas in the fab.
A still further concern is the potential for injury to fab operators or harm to the workpieces in the pods as a result of a pod falling off of the storage nests or the robotic transport gripper. This may occur as a result of seismic activity or, in the event of pod transport, upon failure or breakage of the pod handle by which it is being transported.
It is therefore an advantage of the present invention to prevent injury to operators as a result of impact with the moving portions of the pod transport assembly or pod within the human/automation common area.
It is another advantage of the present invention to prevent damage to workpieces within a pod as a result of contact with fab operators within the human/automation common area.
It is a further advantage of the present invention to allow maintenance to be performed on a tool without risk of injury to the maintenance operator or damage to pods passing through the maintenance area.
It is a still further advantage of the present invention to allow tools within a tool bay to be maintained, removed, replaced and/or rearranged without affecting pod delivery to other areas within the tool bay.
It is another advantage of the present invention to prevent injury to fab operators and damage to workpieces by providing a system for preventing pods from falling in the event of seismic activity or failure of the pod handle by which a pod is being transported.
It is a further advantage of the present invention to provide redundant hardware and software systems for preventing the transport assembly from entering in the human/automation common area of a tool when an operator is present.
It is a still further advantage of the present invention to provide redundant hardware and software systems for preventing the transport assembly from traveling at high speeds in the human/automation common area.
These and other advantages are provided by a system for safeguarding fab operators and workpieces such as semiconductor wafers from harm as the workpieces are transported between tools and storage nests by a workpiece transport assembly. The safeguarding system comprises a transport system in which all high speed horizontal movement of the transport assembly occurs above 2135 mm and out of the human/automation common areas. Thus a pod gripper may be quickly located at the desired horizontal position without danger of collision between operator and transport assembly. The gripper is affixed to a mast assembly so that, once the gripper of the transport assembly is located at the proper horizontal position, the mast assembly lowers down to the desired height for pod transfer. The mast assembly allows pod transfer from the side of a nest. It is a further feature of the transport assembly that the stationary horizontal rails of the assembly are located above 3500 mm. Thus, any tool conforming to the SEMI standard E72 may be maintained, removed, replaced and/or rearranged without interference with the rails and without affecting pod delivery to other areas within the tool bay.
The safeguarding system according to the present invention further includes a light curtain comprised of a sensor array in front of each of the tools in the human/automation common areas. Each of the respective light curtains at the front of the tools are able to sense when an operator is working at a load port, as a result of portions of the operator""s body interrupting one or more of the sensors forming the curtain. The information from the light curtain is forwarded to the circuit providing power to the transport assembly actuators, and is also forwarded to a system host computer.
Each of the tools further includes a safe zone sensor beam comprised of a sensor mounted at each tool above the human/automation common area, at for example a height of approximately 2135 mm. Whenever the mast assembly lowers down into the human/automation common areas in front of a tool, the mast assembly or pod carried thereby will interrupt the safe zone sensor which then transmits this information to the circuit providing power to the transport assembly and to the system host computer.
The safeguarding system in accordance with the present invention includes a redundant system for preventing the transport assembly for entering into the human/automation common area. First, when a light curtain senses the presence of an operator at tool, the system host computer includes a software subroutine that prevents the mast assembly from entering into the human/automation common area of that tool. Additionally, the safeguarding system includes a plurality of hardwired interlock circuits which utilize the information from the light curtain and safe zone sensor beam so that, if at any time both an operator and the transport assembly occupy the same human/automation common area at tool, power is cut to the transport mechanism to thereby prevent injury to the operator and harm to the workpieces within the pods.
The information from the light curtain and safe zone sensor beam is also utilized by the system host computer so that, upon cutting the power to the transport assembly, the system host computer stores the current position and target position of the transport assembly. Upon clearing the conflict at a tool, the system host computer checks to ensure that the location of the transport assembly is the same as when power was cut (via independently powered encoders on the transport assembly), and reinitiates movement of the transport assembly.
The safeguarding system additionally includes a redundant system for ensuring that the transport assembly does not exceed predetermined horizontal and vertical speeds when traveling within the human/automation common areas. In particular, the horizontal and vertical drives of the transport assembly each include encoders which allow the system host computer to monitor the horizontal and vertical location and speed of the mast assembly. The host computer uses the information from the encoders to provide closed loop servo control of the horizontal and vertical drives to ensure that the mast assembly does not exceed the predetermined horizontal or vertical travel speeds when positioned in the human/automation common areas. Additionally, the safeguarding system according to the present invention includes a hardwired velocity circuit which includes a tachometer for monitoring the horizontal and vertical drives independently of the encoders. If the tachometer ever measures a horizontal or vertical travel speed in the human/automation common areas above the predetermined travel speed, power is cut to the transport assembly.
The safeguarding system according to the present invention further includes safety shields for physically closing off the areas between tools which include nests in the human/automation common areas. Safety shields maybe vertically oriented panels that extend down to or near the floor and extend up to a height of approximately 2135 mm. This feature prevents an operator from inadvertently or advertently accessing an area between tools which is also accessed by the transport assembly.